An LCD device includes an LCD panel and a backlight module. A boost circuit is often used in a conventional backlight driving circuit employing an LED as a backlight module. As shown in FIG. 1, when Q1 is turned on, L1 stores the energy; when Q1 is turned off, L1 releases the energy in a reverse direction, which makes an output voltage to be higher than an input voltage, and a current through the LEDs is kept constant by using a constant current module: a constant current is achieved by controlling an impedance of Q2, which enables a voltage of R1 to be equal to a reference voltage Vref. The relationship between the input voltage and the output voltage is:
Vo/Vin=1/(1−D), D is a ratio of a Q1 turn-on time to a cycle period.
After Vin is converted by DC/DC, and then is converted by the boost circuit, the efficiency is low.